Distribution of Urophora Galls in Flower Heads of Diffuse and Spotted Knapweed in British Columbia

Abstract

SUMMARY (1) The distribution of two gall flies, Urophora affinis and U. quadrifasciata, among flower heads of diffuse and spotted knapweed was determined during a population explosion following their introduction to Canada as biological control agents. (2) The distribution of Urophora quadrifasciata was more clumped than that of U. affinis and galls of both flies had more clumped distributions in seed heads of spotted knapweed, which has larger heads and more synchronous bud development than diffuse knapweed. Similar patterns are observed in European populations. (3) On diffuse knapweed Urophora quadrifasciata galls were more randomly distributed in peak populations and U. affinis galls have been more contagiously distributed since the post-introduction peak. (4) Apparently the equilibrium density of flies is not maintained through adjustment of ovipositional behaviour, but is more likely mediated through the response of the plant to fly attack, and by environmental factors which influence the phenology of bud development and fly emergence. (5) The introduction of these two closely related flies has led to greater seed destruction than would have been achieved by either species alone.